The project got its start in an unscientific place: the airport security lineup. Like most travelers, Manijeh Razeghi, a professor at Northwestern University's McCormick School of Engineering and Applied Science, was concerned with both the delays in the process and its accuracy. The technology to safely and easily inspect items for hazardous substances is expensive and bulky, so much of it is underused, Razeghi said. The same concerns — time, reliability and cost — are found in medical diagnostics, tumor detection and package inspection. She wanted to come up with "something useful that can overcome these basic limitations and allow terahertz technology to truly become pervasive in order to make everyone's life a little safer and easier."

Coherent terahertz radiation historically has been very difficult to generate, and the search for a compact easy-to-use source continues today. Existing terahertz sources are large multicomponent systems that may require complex vacuum electronics, external pump lasers and/or cryogenic cooling. A single-component device that does not have these limitations could enable next-generation terahertz systems.
(a) Schematic of an integrated dual-period distributed feedback quantum cascade laser. λ1 and λ2 represent the mid-infrared wavelengths. λ represents the terahertz wavelength. (b) The room-temperature terahertz emission spectra at different operating currents, showing stable single-mode operation. Courtesy of Manijeh Razeghi, Northwestern University.
Razeghi and her group at the Center for Quantum Devices have addressed two key issues that have limited the usefulness of initial demonstrations of mid-IR difference-frequency generation of terahertz radiation: By increasing the power and the beam quality of the mid-IR pumps, the terahertz power has been significantly increased by more than a factor of 30 to approximately 10 µW.

The researchers also incorporated a dual-wavelength diffraction grating within the laser cavity to create single-longitudinal-mode mid-IR sources, which in turn led to very narrow linewidth terahertz emission near 4 THz. The terahertz spectrum is extremely stable with respect to current and temperature, which could make the device valuable as a local oscillator for low-light-level receivers such as those needed for astronomical applications.

The work appeared online Sept. 27 in Applied Physics Letters (doi: 10.1063/1.3645016).

To realize the technology's potential, the team will have to increase the power and efficiency at room temperature and also explore on-chip tunability.
Work in this area was partially supported by DARPA, and Razeghi would like to acknowledge the interest and support of Scott Rodgers of DARPA and Tariq Manzur of the Naval Undersea Warfare Center.

"When we started mid-infrared lasers, we started with a few microwatts," she said. "With interest and funding from DARPA, we increased the output power to 120 W. We are very proud of our terahertz demonstration, but the funding is limited at present."